1. Field of the Invention
This invention relates to an improvement in an exposure control device to be disposed within a camera such as video camera or the like.
2. Description of the Related Art
The degree of exposure in a video camera is determined by exposure time and a diaphragm aperture. Generally, the exposure time is one field period, which is 1/60 sec in the NTSC system and 1/50 sec in the PAL system. Therefore, exposure control is generally performed by adjusting the diaphragm of an optical system. In cases where the camera is provided with an image sensor which is capable of acting as an electronic shutter, the exposure time is controllable by controlling the electronic shutter.
As apparent from the foregoing, the exposure control device of this kind is arranged to perform feedback control over the diaphragm of the optical system or the electronic shutter of the image sensor, or an AGC circuit or the like, inserted in an image signal route in such a way as to keep a video signal level unvarying. FIG. 1 shows an example of the conventional exposure control device. The illustration includes a lens 1; an exposure control circuit 2 which is arranged to control the degree of exposure by controlling a diaphragm, or an electronic shutter in case that the video camera is using a solid-state image sensor; the image sensor 3; a signal level detecting circuit 4 which generates an exposure control signal; a reference power source 5 which generates a level signal corresponding to a desired exposure control value; and a video signal output terminal 6.
An object's image which passes through the lens 1 comes to the image sensor 3 to be formed on the image sensing plane of the image sensor 3. The image sensor 3 converts the image into a video signal. The video signal is supplied to the signal level detecting circuit 4. Further, the video signal is supplied also to a signal processing circuit which is not shown but is arranged to form a television video signal. The image is thus converted into a television video signal in conformity to the standard specifications of the NTSC or PAL system or the like. Meanwhile, the signal level detecting circuit 4 forms an exposure control signal on the basis of the video signal received. The exposure control signal is supplied to the exposure control circuit 2. Then, the exposure control circuit 2 uses this exposure control signal as a measure for an exposure. The circuit 2 causes the signal to coincide always with the signal of a given level value coming from the power source 5 which generates the level signal corresponding to the desired exposure control value.
The signal level detecting circuit 4 has been arranged to measure light by one of the following different methods including (1) full plane average light measurement and (2) peak light measurement. However, these methods have the following drawbacks respectively:
In the light measuring method (1): The light is measured by the average value of luminance obtained on the full image (sensing) plane. Therefore, if a large portion of the image plane is occupied by a bright matter such as a light source, a main photographed object would appear in an excessively dark state. If the background becomes dark, the main object would appear in a too bright state failing to show its details.
In the case of the light measuring method (2): Light measurement is performed according to the peak value of the luminance obtained on the image plane. Therefore, the whole image plane would become excessively dark if some high luminance matter comes to occupy even a small portion of the image plane.
The drawbacks of the methods (1) and (2) can be mitigated to a certain degree by a center-weighted light measuring method (3). In the case of this method (3), however, the exposure changes when the main object moves out of the middle part, or the light measuring spot, of the image sensing plane either due to a slight movement of the camera or due to the movement of the object. In that event, the brightness of the image plane changes to give an unnatural impression. This drawback is salient especially where there is an excessive contrast between the object and the background. Another drawback of this light measuring method lies in that in cases where the size of the object within the image plane is considerably smaller than that of a light measuring area which occupies a given portion of the image plane, exposure control on the basis of an image signal obtained from the light measuring area would be affected by the background. Conversely, if the object is much larger than the size of the light measuring area within the image plane, exposure control cannot be adequately accomplished with the image signal of the light measuring area as it would be impossible to obtain a sufficient amount of information for the whole image plane.
Further, even if an exposure control device is arranged to be capable of coping with changes taking place in the position and size of the object's image within the image plane, any object other than the main photographed object would likely be disregarded. Therefore, while the device may give the optimum degree of exposure for the main object, the exposure is not always the best for the whole image plane.